Your analysis of the situation is correct. It is impossible to plan that type of military movement within 24 hours. Clearly it was planned a while ago. I think that the Russian president did not want to send any armed forces during the Olympic games. As several predicted, as soon as the closing ceremonies were over he once again showed his true colours.
I would like to add something else. This is often being described as a conflict between Russians and the Ukrainians, or between the extremists and all the others. I think that the real reason the Russian president sent in the army was because of what happened in the Maidan. Everything started with a political issue, the direction a country was going to take, but it turned into a will on the part of the population to live in a democratic country without abuse nor corruption. Whether they live in an eastern, western, northern or southern country, any normal family wants to live in those conditions. I think that what the Russian president felt was that this will, which turned into a spectacular victory for the Maidan, could also surface in Russia.